Overflow downdraw glass forming method and apparatus

ABSTRACT

The present invention discloses improved methods and apparatus for forming sheet glass. In one embodiment, the invention introduces a counteracting force to the stresses on the forming structure in a manner such that the thermal creep which inevitably occurs has a minimum impact on the glass flow characteristics of the forming structure.

REFERENCE TO RELATED APPLICATIONS

This application claims an invention that was disclosed in ProvisionalApplication Ser. No. 60/751,419, filed Dec. 15, 2005, entitled “OVERFLOWDOWNDRAW GLASS FORMING METHOD AND APPARATUS”. The benefit under 35 USC§119(e) of the United States provisional application is hereby claimed,and the aforementioned application is hereby incorporated herein byreference.

This application is a continuation in part of copending U.S. applicationSer. No. 11/006,251, filed on Dec. 7, 2004, entitled “OVERFLOW DOWNDRAWGLASS FORMING METHOD AND APPARATUS”, which is a divisional applicationof U.S. Pat. No. 6,889,526, filed Aug. 8, 2002, issued May 10, 2005,entitled “OVERFLOW DOWNDRAW GLASS FORMING METHOD AND APPARATUS”, whichclaims an invention that was disclosed in one of the followingprovisional applications:

-   -   1) Provisional Application No. 60/310,989, filed Aug. 8, 2001,        entitled “SHEET GLASS FORMING DEVICE”;    -   2) Provisional Application No. 60/316,676, filed Aug. 29, 2001,        entitled “SHEET GLASS FORMING DEVICE”;    -   3) Provisional Application No. 60/318,726, filed Sep. 12, 2001,        entitled “SHEET GLASS FORMING APPARATUS”;    -   4) Provisional Application No. 60/318,808, filed Sep. 13, 2001,        entitled “SHEET GLASS FORMING APPARATUS”;    -   5) Provisional Application No. 60/345,464, filed Jan. 3, 2002,        entitled “SHEET GLASS FORMING APPARATUS”; and    -   6) Provisional Application No. 60/345,465, filed Jan. 3, 2002,        entitled “SHEET GLASS FORMING APPARATUS”.

The benefit under 35 USC §119(e) of the United States provisionalapplications is hereby claimed, and the aforementioned applications andpatents are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the manufacture of glass sheet and,more particularly, to glass sheet used for the production of LCD displaydevices that are widely used for computer displays.

2. Description of Related Art

The glass that is used for semiconductor powered display applications,and particularly for TFT/LCD display devices that are widely used forcomputer displays, must have very high surface quality to allow thesuccessful application of semiconductor type material. Sheet glass madeusing the apparatus of U.S. Pat. No. 3,338,696, assigned to Corning,Inc., makes the highest quality glass as formed and does not requirepost-processing. The Corning patent makes glass by a manufacturingprocess termed: “The Overflow Process”. Glass made using other processesrequires grinding and/or polishing and thus does not have as fine asurface finish. The glass sheet must also conform to stringent thicknessvariation and warp specification. The fine surface finish is formed fromvirgin glass primarily from the center of the glass stream. This glasshas not been in contact with foreign surfaces since the stirringoperation.

The teachings of U.S. Pat. No. 3,338,696 are still the state of the artas practiced today. However, the apparatus has limitations.

A major drawback of the apparatus of “The Overflow Process” is that,even though it makes excellent glass over most of the surface, thesurface of the glass sheet nearest the inlet is composed of glass thathas flowed in proximity to the feeding pipe surfaces and therefore issubject to lower quality.

Another drawback of the apparatus of “The Overflow Process” is that,even though its makes excellent glass during stable operatingconditions, it recovers from transient conditions very slowly. This iscaused in part by quiescent zones of glass flow in the pipes conductingthe glass from the stirring device to the apparatus when these pipes aredesigned using traditional practice. During unintended processtransients, these quiescent zones slowly bleed glass of a previousmaterial composition into the main process stream of glass causingdefects. These defects eventually subside when the process stabilizes;however, there is a period of time where the quality of the glass sheetis substandard.

Yet another drawback of the apparatus of “The Overflow Process” is thelimited means for controlling the thickness of the formed sheet. Theselective cooling of the glass with respect to width as the sheet isformed is not provided in current practice. The radiant heat losses fromthe lower inverted slope portion of the forming structure are notcontrolled. This lack of control can have a significant impact on theflatness (warp) of the formed sheet.

The thickness control system of U.S. Pat. No. 3,682,609 can compensatefor small thickness errors, but it can only redistribute the glass overdistances on the order of 5-10 cm.

A further drawback of the apparatus of “The Overflow Process” is thatsurface tension and body forces have a major effect on the molten glassflow down the external sides of the forming apparatus causing the sheetto be narrower than the forming apparatus and the edges of the formedsheet to have thick beads.

U.S. Pat. No. 3,451,798 provides for edge directors which endeavor tocompensate for the surface tension effects but are in reality acorrection for problems created by restricting the forming apparatuscross-section to a single profile on its external surface.

Yet another drawback of the apparatus of “The Overflow Process” is thatthe drawing of the sheet from the bottom of the apparatus has apropensity to have a cyclic variation in sheet thickness. This cyclicthickness variation is a strong function of uncontrolled air currents,which tend to become more prevalent as the equipment ages during aproduction campaign. As the apparatus ages, air leaks develop throughcracks in material and assorted openings caused by differentialexpansion.

A significant drawback of the apparatus of “The Overflow Process” isthat the forming apparatus deforms during a manufacturing campaign in amanner such that the glass sheet no longer meets the thicknessspecification. This deformation is thermal creep of the formingapparatus caused by gravitational forces. This is a primary cause forpremature termination of the production run. This deformation occursover an extended period of time. During this time, the process iscontinuously changing such that process adjustments must be made tocompensate for the sagging of the forming apparatus. This adjustmentactivity leads to loss of salable product.

Therefore, there is a need in the art for an apparatus which overcomesthe shortcomings of the prior art.

SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention, all the glass thatforms the surface of the useful area of the sheet is virgin glass, whichis not contaminated by flow in proximity to a refractory or refractorymetal surface after the stirring operation. In addition, this embodimentsignificantly reduces inhomogeneities in the glass that forms the sheetby eliminating the regions of quiescent flow in the piping between thestirring device and the sheet glass forming apparatus.

In another preferred embodiment, this invention introduces a precisethermal control system to redistribute the flow of molten glass at theweirs which is the most critical area of the forming process. Thisthermal control effectively counteracts the degradation of the sheetforming apparatus which inevitably occurs during a production campaign.

In another embodiment the radiant heat loss from the bottom of theinverted slope of the forming structure is longitudinally adjusted byvarying the width of the exit opening of the forming chamber.

Another preferred embodiment creates a variable external cross-sectionwhich alters the direction and magnitude of the surface tension and bodyforce stresses and thus, reduces the adverse influence of surfacetension and body forces on sheet width.

In an alternative preferred embodiment, the glass is preferentiallycooled across its width to create forming stresses duringsolidification, which ensures that the glass sheet drawn is inherentlyflat.

In a further preferred embodiment, this invention adjusts the internalpressure in each of the major components of the forming apparatus suchthat the pressure difference across any leakage path to the forming zoneapproaches zero. Therefore, air leakage in the apparatus is minimizedeven though cracks and openings exist during initial operation anddevelop during manufacturing.

In yet another preferred embodiment, the invention introduces acounteracting force to the stresses caused by gravitational forces onthe forming structure in a manner such that the thermal creepdeformation (strain) that results from thermal creep has a minimumeffect on the thickness variation of the glass sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the principle parts of “The Overflow Process” glasssheet manufacturing system.

FIG. 2A shows a side view of “The Overflow Process” as known in theprior art.

FIG. 2B shows a cross-section of the glass flow in the downcomer pipeacross lines B-B of FIG. 2A.

FIG. 2C shows a cross-section across lines C-C of FIG. 2A, where theglass flow in the downcomer pipe appears in the sheet for “The OverflowProcess”.

FIG. 3A shows a side view of a surface flow distribution device in apreferred embodiment of the present invention.

FIG. 3B shows a top view of a surface flow distribution device in apreferred embodiment of the present invention.

FIG. 4A shows a side view of a submerged flow distribution device in apreferred embodiment of the present invention.

FIG. 4B shows a top view of a submerged flow distribution device in apreferred embodiment of the present invention.

FIG. 5A shows a side view of “The Overflow Process” in an embodiment ofthe present invention.

FIG. 5B shows the glass flow in the downcomer pipe across lines B-B ofFIG. 5A when a flow distribution device is used.

FIG. 5C shows a cross-section across lines C-C of FIG. 5A, where theglass flow in the downcomer pipe appears in the sheet when a flowdistribution device is used.

FIG. 6 shows a bowl with an inclined axis which diffuses the quiescentflow zone at the bowl nose in a preferred embodiment of the presentinvention.

FIG. 7A shows the top view of a bowl with side inflow which relocatesthe quiescent flow zone from the bowl nose to the bowl side in apreferred embodiment of the present invention.

FIG. 7B shows a side view of FIG. 7A.

FIG. 7C shows the top view of a bowl with side inflow which relocatesthe quiescent flow zone from the bowl nose to a location approximately45 degrees to the side with respect to the centerline of the formingapparatus in a preferred embodiment of the present invention.

FIG. 7D shows a side view of FIG. 7C.

FIG. 8 illustrates a bowl in “The Overflow Process” as known in theprior art.

FIG. 9A shows a downcomer pipe feeding the forming apparatus inlet withminimum quiescent flow in a preferred embodiment of the presentinvention.

FIG. 9B shows a top view of FIG. 9A.

FIG. 9C shows a detail of the downcomer pipe to trough inlet pipeconnection showing the glass flow pattern in a preferred embodiment ofthe present invention.

FIG. 10A shows the flow between the downcomer pipe and the formingapparatus inlet in “The Overflow Process” as known in the prior art.

FIG. 10B shows a top view of FIG. 10A.

FIG. 10C shows a detail of the downcomer pipe to trough inlet pipeconnection showing the glass flow pattern as known in the prior art.

FIG. 11A shows the principle parts of a typical “Overflow Process”manufacturing system.

FIG. 11B shows a section of FIG. 11A.

FIG. 12A illustrates a side view of the glass flowing through theforming structure.

FIG. 12B shows a section through the center of the forming structure ofFIG. 12A showing the cooling zones.

FIG. 13A shows a revised single heating chamber muffle design in apreferred embodiment of the present invention.

FIG. 13B shows a section of FIG. 13A.

FIG. 14A shows air cooling tubes to affect localized cooling to themolten glass as it passes over the weirs in a preferred embodiment ofthe invention.

FIG. 14B shows a section of FIG. 14A.

FIG. 15A shows a muffle with multiple heating chambers in a preferredembodiment of the invention.

FIG. 15B shows a section of FIG. 15A.

FIG. 16A shows radiant coolers which affect localized cooling to themolten glass as it passes over the weirs in a preferred embodiment ofthe invention.

FIG. 16B shows a section of FIG. 16A.

FIG. 17A illustrates how the prior art forming structure design deformsas a result of thermal creep.

FIG. 17B shows another view of FIG. 17A.

FIG. 18A shows the forming structure support system as known in theprior art.

FIG. 18B shows another view of FIG. 18A.

FIG. 18C shows another view of FIG. 18A.

FIG. 18D shows another view of FIG. 18A.

FIG. 19A shows single shaped compression blocks on each end of theforming structure in a preferred embodiment of the present invention.

FIG. 19B shows another view of FIG. 19A.

FIG. 19C shows another view of FIG. 19A.

FIG. 19D shows another view of FIG. 19A.

FIG. 20A shows a single shaped compression block on one end of theforming structure and multiple shaped compression blocks on the otherend in a preferred embodiment of the present invention.

FIG. 20B shows another view of FIG. 20A.

FIG. 20C shows another view of FIG. 20A.

FIG. 20D shows another view of FIG. 20A.

FIG. 21A shows a forming structure design as known in the prior art.

FIG. 21B shows a top view of FIG. 21A.

FIG. 21C shows a cross-section of the forming structure design shown inFIG. 21A across lines C-C.

FIG. 21D shows a cross-section of the forming structure design shown inFIG. 21A across lines D-D.

FIG. 21E shows a cross-section of the forming structure design shown inFIG. 21A across lines E-E.

FIG. 21F shows a cross-section of the forming structure design shown inFIG. 21A across lines F-F.

FIG. 21G shows a cross-section of the forming structure design shown inFIG. 21A across lines G-G.

FIG. 22A shows a reduced inverted slope at each end of the formingstructure in a preferred embodiment of the present invention.

FIG. 22B shows a top view of FIG. 22A.

FIG. 22C shows a cross-section of the forming structure design shown inFIG. 22A across lines C-C.

FIG. 22D shows a cross-section of the forming structure design shown inFIG. 22A across lines D-D.

FIG. 22E shows a cross-section of the forming structure design shown inFIG. 22A across lines E-E.

FIG. 22F shows a cross-section of the forming structure design shown inFIG. 22A across lines F-F.

FIG. 22G shows a cross-section of the forming structure design shown inFIG. 22A across lines G-G.

FIG. 23A shows an alternate embodiment of the present invention withfurther modified ends.

FIG. 23B shows a top view of FIG. 23A.

FIG. 23C shows a cross-section of the forming structure design shown inFIG. 23A across lines C-C.

FIG. 23D shows a cross-section of the forming structure design shown inFIG. 23A across lines D-D.

FIG. 23E shows a cross-section of the forming structure design shown inFIG. 23A across lines E-E.

FIG. 23F shows a cross-section of the forming structure design shown inFIG. 23A across lines F-F.

FIG. 23G shows a cross-section of the forming structure design shown inFIG. 23A across lines G-G.

FIG. 24A shows an alternate embodiment of the present invention with thepotential for increased structural stiffness.

FIG. 24B shows a top view of FIG. 24A.

FIG. 24C shows a cross-section of the forming structure design shown inFIG. 24A across lines C-C.

FIG. 24D shows a cross-section of the forming structure design shown inFIG. 24A across lines D-D.

FIG. 24E shows a cross-section of the forming structure design shown inFIG. 24A across lines E-E.

FIG. 24F shows a cross-section of the forming structure design shown inFIG. 24A across lines F-F.

FIG. 24G shows a cross-section of the forming structure design shown inFIG. 24A across lines G-G.

FIG. 25A shows a forming structure with a continuously curvedparabolically shaped convex upward root with constant inverted slopeangle which solidifies the center glass before the edge glass in apreferred embodiment of the present invention.

FIG. 25B shows an end view through section B-B of FIG. 25A.

FIG. 25C shows a top view of FIG. 25A.

FIG. 25D shows a section view through section D-D of FIG. 25A.

FIG. 26A shows a forming structure with a continuously curvedparabolically shaped convex upward root with variable inverted slopeangle which solidifies the center glass before the edge glass in apreferred embodiment of the present invention.

FIG. 26B shows an end view through section B-B of FIG. 26A.

FIG. 26C shows a top view of FIG. 26A.

FIG. 26D shows a section view through section D-D of FIG. 26A.

FIG. 27A shows the forming structure of FIGS. 25A through 25D containedin the heating chamber of a muffle with movable bottom doors at itsbottom to control radiation heat loss.

FIG. 27B shows a section view through section B-B of FIG. 27A.

FIG. 27C shows a section view through section C-C of FIG. 27A in apreferred embodiment of the present invention.

FIG. 27D shows a section view through section D-D of FIG. 27A in apreferred embodiment of the present invention.

FIG. 27E shows a section view through section E-E of FIG. 27A whichrepresents the prior art shape of the movable bottom doors.

FIG. 28A illustrates the cooling process in “The Overflow Process” glasssheet forming system as known in the prior art.

FIG. 28B shows a section of FIG. 28A.

FIG. 29A shows how the pressure in the muffle zone may be controlled tominimize leakage in a preferred embodiment of the present invention.

FIG. 29B shows a section of FIG. 29A.

FIG. 30A shows how the pressure in the muffle door zone may becontrolled to minimize leakage in a preferred embodiment of the presentinvention.

FIG. 30B shows a section of FIG. 30A.

FIG. 31A shows how the pressure in the transition zone may be controlledto minimize leakage in a preferred embodiment of the present invention.

FIG. 31B shows a section of FIG. 31A.

FIG. 32A shows how the pressure in the annealer and pulling machine zonemay be controlled to minimize leakage in a preferred embodiment of thepresent invention.

FIG. 32B shows a section of FIG. 32A.

FIG. 33A is a side view of glass flowing in and over the formingstructure of the overflow process.

FIG. 33B is an inflow end view of the glass and forming structure shownin FIG. 33A.

FIG. 33C is a far end view of the glass and forming structure shown inFIG. 33A.

FIG. 33D is a top view of the glass and forming structure shown in FIG.33A.

FIG. 34A is an illustration of the thermal creep deformation of theglass forming structure under the load of its own weight.

FIG. 34B is an illustration of the thermal creep deformation of theglass forming structure under an applied load that minimizes verticaldeformation.

FIG. 34C is an illustration of the thermal creep deformation of theglass forming structure under excessive applied load.

FIG. 34D is an illustration of the thermal creep deformation of theglass forming structure under an applied load that minimizes verticaldeformation over the extended period of a production campaign.

FIG. 35 is a graph of the thermal creep material characteristics of therefractory material used in the forming structure.

FIG. 36 shows the deformation of the prior art linear FEA of the formingstructure deformation with no corrective forces.

FIG. 37 shows the deformation of the prior art linear FEA of the formingstructure deformation with corrective forces per U.S. Pat. No.3,519,411.

FIG. 38 shows the deformation of the linear FEA of the forming structuredeformation with corrective forces for virtually no sagging of theforming structure.

FIG. 39A is a side view of the grid, with dimensions in the metricsystem, used in the FEA.

FIG. 39B is an end view of the grid, with dimensions in the metricsystem, used in the FEA.

FIG. 39C is an end view of the forming structure showing where the forcewas applied to the bottom portion of the forming structure in the FEA.

FIG. 39D shows the assumed temperature distribution of the cross-sectionof the forming structure.

FIG. 40 shows the deformation of nonlinear FEA of the forming structuredeformation with corrective forces for virtually no sagging of theforming structure as predicted by linear FEA.

FIG. 41 shows the deformation of nonlinear FEA of the forming structuredeformation with corrective forces for virtually no sagging of theforming structure.

FIG. 42A illustrates the prior art glass forming structure support andcompression system.

FIG. 42B shows a sectional view of FIG. 42A.

FIG. 42C shows a partial view of FIG. 42A.

FIG. 42D shows a sectional view of FIG. 42A.

FIG. 43A shows a forming structure support system involving supportblocks for the weight of the forming structure at each end andindividual compression blocks and force applicators at each end.

FIG. 43B shows a sectional view of FIG. 43A.

FIG. 43C shows a partial view of FIG. 43A.

FIG. 43D shows a sectional view of FIG. 43A.

FIG. 44A shows a forming structure support system involving support andcompression blocks and force applicators at each end.

FIG. 44B shows a sectional view of FIG. 44A.

FIG. 44C shows a partial view of FIG. 44A.

FIG. 44D shows a sectional view of FIG. 44A.

FIG. 45A shows a forming structure support system involving support andcompression blocks, force applicators at each end, and a sealing forceapplicator at the far end.

FIG. 45B shows a sectional view of FIG. 45A.

FIG. 45C shows a partial view of FIG. 45A.

FIG. 45D shows a sectional view of FIG. 45A.

FIG. 46A is a section through the downcomer pipe to the inlet pipejunction where the downcomer pipe is immersed below the glass freesurface.

FIG. 46B is a detail of the glass flow in a section of FIG. 46A.

FIG. 47A is a section through the downcomer pipe to the inlet pipejunction where the downcomer pipe is substantially above the glass freesurface.

FIG. 47B is a detail of the glass flow in a section of FIG. 47A.

FIG. 48A is a section through the downcomer pipe to inlet pipe junctionwhere the downcomer pipe is the same distance as the diameter of theinlet pipe above the glass free surface.

FIG. 48B is a detail of the glass flow in a section of FIG. 48A.

FIG. 49A shows heaters at the downcomer pipe to the inlet pipe junction.

FIG. 49B is a detail of the glass flow in a section of FIG. 49A.

FIG. 49C is a partial top view of FIG. 49A.

FIG. 49D is a detail of a typical sealing block as used in FIGS. 49A,49B, and 49C.

FIG. 50A is a section through the downcomer pipe to the inlet pipejunction where a conical section is added to the inlet pipe.

FIG. 50B is a detail of the glass flow in a section of FIG. 50A.

FIG. 51 shows a device that precisely controls the cooling air mass flowinto and out of the muffle door chamber to minimize the leakage from themuffle doors in an embodiment of the present invention.

FIG. 52 shows a device that controls the forced convective cooling ofthe glass flowing off the sheet forming structure by preciselycontrolling the cooling air mass flow into and out of the muffle doorchamber in an embodiment of the present invention.

FIG. 53 shows the cross-section of the muffle door in FIG. 52 as it fitsinto the sheet forming apparatus.

DETAILED DESCRIPTION OF THE INVENTION

The flow dynamics in all embodiments of this invention are such that theoutside surfaces of the glass sheet are formed from thoroughly mixedvirgin glass that comes from the center of the glass stream flowing intothe forming apparatus and thus has not contacted a refractory orrefractory metal surface. This produces the highest possible surfacequality. This pristine surface is essential for the manufacture ofLCD/TFT semiconductor display devices. In addition, the flow dynamics inall embodiments of this invention are such that the flow rate of moltenglass to the forming wedge at the bottom of the forming structure issubstantially uniform over its width.

The glass “Sheet Forming Apparatus” normally designed for use in “TheOverflow Process” (U.S. Pat. No. 3,338,696) relies on a specificallyshaped forming structure to distribute the glass in a manner to form asheet of a uniform thickness. The basic shape of this forming structureis described in detail in U.S. Pat. No. 3,338,696. The sheet glassforming process is conducted at elevated temperatures, typically between1150° C. and 1275° C. At these temperatures, the materials used forconstruction of the forming structure exhibit a property called thermalcreep, which is deformation of the material caused by applied stress atelevated temperatures. Thus, the forming structure deforms under thestress caused by its own weight and the stress caused by the weight andhydrostatic pressure of the molten glass in and on the formingstructure.

As used herein, stress is the magnitude of the force within the formingstructure, while strain is the deformation of the forming structure.

The concern for the structural integrity of the forming structure wasaddressed by Cortright in U.S. Pat. No. 3,519,411. The refractorymaterial from which the forming structure is made has a high strength incompression and a low strength in tension. To prevent fracture of theforming structure, a compressive force was applied to the bottom portionof each end of the forming structure with the objective “to alleviatethe undesirable effects of tensile stresses in a refractory sheet glassforming member”. This force was determined by a static closed formstress analysis in U.S. Pat. No. 3,519,411, because Finite ElementAnalysis (FEA) was not a technology used by those skilled in art in theglass industry at that time.

U.S. Pat. No. 3,437,470 (Overman) also provides a design to negate theeffect of gravity on the structural integrity of the forming structure.In both these patents, there was no documented concern or informationabout thermal creep of the forming structure as it relates to thethickness of the glass sheet produced.

U.S. Pat. Nos. 6,748,765, 6,889,526, and 6,895,782 and U.S. patentapplication Ser. Nos. 10/826,097, 11/011,657, 11/060,139 and 11/184,212have been filed by the present applicant addressing, in different ways,the various problems of thermal creep in the Overflow Process. Thesepatents and patent applications are herein incorporated by reference.They describe handling thermal creep as a linear process. While thesemethods and apparatuses work as described, there are significantadvantages to be gained by considering the nonlinear behavior of thermalcreep. The present invention enhances the claims and technology of theabove referenced patents and patent applications by considering thenonlinear thermal creep characteristics of the refractory formingstructure with respect to temperature and stress.

The invention introduces an accurate calculation of the counteractingforce to the gravitational force on the forming structure in a mannersuch that the thermal creep, which inevitably occurs, has virtually noimpact on the glass flow characteristics of the forming structure. Theinvention is designed such that this counteracting force is adequate toovercome the nonlinear aspects of thermal creep of the refractorymaterial and is maintained through an extended period of the productioncampaign. Thus, sheet glass may be manufactured to the originalspecification for a longer time with the same forming structure andprocess parameters.

Referring to FIGS. 1, 11A and 11B, a typical “Overflow Process”manufacturing system (1) is shown. The glass (10) from the meltingfurnace (2) and forehearth (3), which must be of substantially uniformtemperature and chemical composition, feeds a stirring device (4). Thestirring device (4) thoroughly homogenizes the glass. The glass (10) isthen conducted through a bowl inlet pipe (5), into a bowl (6), and downinto the downcomer pipe (7), through the joint (14) between thedowncomer pipe (7) and the forming apparatus inlet pipe (8), to theinlet of the overflow forming structure (9). While flowing from thestirring device (4) to the forming structure (9), the glass (10),especially that which forms the sheet surface, must remain homogeneous.The normal purpose of the bowl (6) is to alter the flow direction fromhorizontal to vertical and to provide a means for stopping the flow ofglass (10). In some apparatus configurations, a needle (13) is providedto stop glass flow. The normal function of the joint (14) between thedowncomer pipe (7) and the trough inlet pipe (8) is to allow for removalof the sheet glass forming apparatus for service as well as a means ofcompensation for the thermal expansion of the process equipment.

The molten glass (10) from the melting furnace and forehearth, whichmust be of substantially uniform temperature and chemical composition,enters the forming apparatus through the inlet pipe (8) to a trough(129) located at the top of the sheet forming structure (9). The inletpipe (8) is preferably shaped to control the velocity distribution ofthe incoming molten glass flow. The glass sheet forming apparatus, whichis described in detail in both U.S. Pat. No. 3,338,696 and U.S. Pat. No.6,748,765, herein incorporated by reference, is a wedge shaped formingstructure, or forming structure (9). Straight sloped weirs (115),substantially parallel with the pointed edge of the wedge (116), formeach side of the trough (129). The bottom (117) of the trough (129) andsides (118) of the trough (129) are contoured in a manner to provideeven distribution of glass to the top of each side weir (115). The glassthen flows over the top of each side weir (115), down each side of thewedge shaped forming structure (9), and joins at the pointed edge of theroot (116), to form a sheet of molten glass (11). The sheet of moltenglass (11) is then cooled as it is pulled off the root (116) by pullingrollers (111) to form a solid glass sheet (12) of substantially uniformthickness. Edge rollers (110) may also be used to draw the molten glasssheet (11). In the prior art, the forming structure (9) is encasedwithin a rectangular shaped muffle (112), the purpose of which is tocontrol the temperature of the forming structure (9) and the moltenglass (10). It is prior art practice to maintain a constant temperaturein the muffle chamber (113) surrounding the forming structure (9). Themuffle (112) is heated by heating elements (138) in heating chamber(119), which is encased in the insulated structure (133). Cooling theglass as it transitions from the molten state to the solid state must becarefully controlled. This cooling process starts on the lower part ofthe forming apparatus (9) just above the root (116), and continues asthe molten glass sheet passes through the muffle door zone (114). Themolten glass is substantially solidified by the time it reaches thepulling rollers (111). The molten glass forms a solid glass sheet (12)of substantially uniform thickness.

A primary element in the overflow process sheet forming apparatus is theforming structure (9). The forming structure (9) is also known by manyother names by those skilled in the art, including, but not limited to,forming trough, forming wedge, forming member, forming device, formingblock, trough, pipe, isopipe, and fusion pipe.

Altering Glass Flow Distribution

Referring also to FIG. 2 through FIG. 10, a preferred embodiment of thepresent invention alters the flow path at the inlet of the sheet glassforming apparatus to improve surface quality. It also facilitates moreuniform flow of glass through the piping that conducts the glass fromthe stirring device to the sheet glass forming apparatus.

U.S. Pat. No. 3,338,696 considers only the glass flow within the formingstructure. U.S. Pat. No. 3,338,696 also claims that the entire sheetsurface is formed from virgin glass, which has not been adverselyeffected by contact with a foreign surface. This is not entirely correctas some of the glass which forms the sheet on the inlet end of thetrough has flowed in contact with the downcomer pipe front surface. Aflow distribution device is added at the trough inlet in this inventionto ensure that all of the usable sheet surface is formed from virginglass. The piping system between the glass stirring device and the glasssheet forming apparatus is modified from traditional practice in thebowl and at the connection between the downcomer pipe and the formingapparatus inlet pipe. The flow through the bowl is altered, eithereliminating or relocating the quiescent flow zone that normally forms atthe front top surface of the bowl. The downcomer pipe is not submergedin the forming apparatus inlet pipe glass thus eliminating the quiescentflow zone between the pipes.

FIGS. 2A through 2C illustrate where the glass (10) flowing in thedowncomer feed pipe (7) ends up in the formed glass sheet in the priorart “Overflow Process”. The glass flow in proximity to the back surface(21) of the downcomer pipe (7) ends up in the center of the drawn sheet.The flow (23) in proximity to the front surface of the downcomer pipe(7) is distributed over the entire glass surface; however, it is mostconcentrated on the approximate one third of the sheet at the inlet end.This surface glass (23) is subject to disruption by the downcomer pipesurface and by the glass in the quiescent zones in the bowl (6) and atthe downcomer pipe (7) to the inlet pipe (8) connection (14). Thesurface of the remaining substantially two thirds of the sheet is formedfrom virgin interior glass (22). Two other portions of the glass flow(24) which are symmetrically offset from the front surface at an angleof approximately 45 degrees end up forming the near end unusable edgesection (25) at the inlet end of the sheet. Another portion (26)centered at an angle of approximately 180 degrees proceeds to the farend of the unusable edge section (27).

FIGS. 3A and 3B show an embodiment of the glass sheet forming apparatus(31) with an inflow pipe (8), a flow distribution device (32) located atthe trough inlet surface (which is the subject of this invention), andthe glass sheet forming apparatus body (9). The flow distribution device(32) interrupts the flow on the surface glass and diverts it to thesurface in the edge of the sheet. Glass from the center of the downcomerpipe flow stream then comes to the surface of the forming structure toform the surface of the usable portion of the glass sheet (11). Notethat ten to twenty percent of the sheet at each edge is normallyunusable for various reasons.

FIGS. 4A and 4B show an alternative embodiment of the glass sheetforming apparatus (41), which performs the same function as theembodiment in FIG. 3 except that the surface flow distribution device(42) is located under the surface of the glass (10) and redistributesthe surface flow in a more subtle but equally effective manner. Theglass flow (10) that forms the edge of the sheet flows through thecenter slot (43) in the flow distribution device (42). The glass (whichflows through this center slot) is the glass that has been in proximityto the front surface of the downcomer pipe. Glass from the center of thedowncomer pipe then flows to the forming structure surface to form thesurface of the usable portion of the sheet (11). Other glass that flowsin proximity to the surface of the downcomer pipe remains submerged.

FIGS. 5A through 5C illustrate where the glass (10) flowing in thedowncomer feed pipe (7) ends up in the formed glass sheet for theinventions described in FIGS. 3 and 4. The glass flow to the center ofthe sheet (21) is virtually identical to that in the prior art. However,the flow (52) which forms the outside surface of the formed glass sheetdoes not flow in proximity to the front surface of the downcomer pipe(7). The two portions of the glass flow (24) which are symmetricallyoffset from the front surface at an angle of approximately 45 degreesand which end up forming the unusable edge section (25) at the inlet endof the sheet are substantially unaffected.

FIG. 6 is an embodiment that shows the axis of the bowl (66) inclined atan angle such that the main process stream passes through the front ofthe bowl. This active flow (60) entrains the surface glass (61),overcoming the surface tension forces that would normally create aquiescent zone of glass flow located at the bowl nose (FIG. 8). A needle(13) is present to stop glass flow.

FIGS. 7A through 7D show an embodiment of the present invention where acrossways motion of the glass in the bowl (76) is facilitated by feedingthe glass in the pipe coming from the stirring device to the bowl (75),into the side of the bowl (76) at an angle (74) with respect to thecenterline (73) of the forming apparatus (9). This effectively changesthe flow pattern (70) in the bowl such that the quiescent zone normallylocated at the bowl nose (81, FIG. 8) is moved to the side of the bowl(71). Referring back to FIGS. 2A-2C and 5A-5C, depending on the angle(74) of the flow in the bowl with respect to the centerline (73) of theforming apparatus (9), the glass from the quiescent zone (71) ends up ineither the unusable portion of the inlet edge (25) or is submerged inthe center of the glass sheet (21) instead of on the surface of theglass sheet (23). The glass free surface (72) in the bowl is also shown.

FIG. 8 illustrates the prior art with a bowl (6) which shows thequiescent zone (81) of glass that is located at the front of the bowl(6). This glass is kept in place by a combination of low process streamflow (80) at the front of the bowl and surface tension.

Eliminating Inhomogeneity Defects at the Downcomer Pipe to Inlet PipeJunction

FIGS. 9A through 9C show an embodiment of the present invention wherethe bottom end (94) of the downcomer pipe (7) is located substantiallyabove the glass free surface (90) in the forming apparatus inlet pipe(98). The forming apparatus inlet (98) also has a specific size andshape (92). The vertical distance (93) and the size and shape (92) ofthe forming apparatus inlet (98) is specifically designed to minimizeany zone of quiescent or vortex flow in the glass flow path (91). Thus,the molten glass (10) forms a more homogenous sheet (11). This design isdetermined by solution of the fluid flow equations (Navier-StokesEquations) and by experimental tests.

FIGS. 10A through 10C show a downcomer pipe (7) submerged in the moltenglass surface (100) in the forming apparatus inlet pipe (8) as known inthe prior art. There is a quiescent zone (101) between the two pipes (7)and (8). The glass flow path (103) produces an annular vortex (102) ofglass between the downcomer pipe (7) and the trough inlet pipe (8). Thevortex exchanges little material with the main process stream exceptduring flow transients at which time it produces defects in the glasssheet.

There are three principal homogeneity defects which may be formed at thedowncomer pipe (7) to the inlet pipe (8) junction (14). These defectsare cord defects, seed defects, and devitrification defects.

Cord is best described as strings of glass of different viscosity and/orindex of refraction in the body glass. It is visually evident in antiqueglass as lines or swirls of distortion. It is caused by poor mixing ofthe glass. Cord may be formed from well mixed glass if the glass flow ina region is flowing much slower than the body glass or if it is subjectto surface volatilization. The chemical composition of glass in manymanufacturing operations changes a small amount from day to day. Ifglass from a previous day's production is slowly bled into the glass ofthe present day's production, a small difference in index of refractionor viscosity can produce cord. In the sheet glass process, cord wouldshow as optical distortion caused by refractive index or thicknessvariations. Thickness variations could also affect the quality of thesemiconductor manufacturing process.

Seeds are gaseous inclusions or bubbles in the glass body. When they arelarge, they are called blisters. Seeds are quite common in melted glassand are kept to the minimum required of a particular product by aprocess called fining or refining. The process is normally both chemicaland mechanical and performed in a finer or refiner. Seeds can beproduced in glass after the fining step by electrolysis and fluid flowphenomena. In the sheet glass process, seeds are elongated and show upas visual defects.

Devitrification is the crystallization of the glass. Glass is amorphousmeaning that it is a completely random mixture of molecules. Moltenglass remains amorphous as long as its temperature is above the liquidustemperature. For glasses that are intended to be transparent, when themolten glass is cooled quickly from above the liquidus temperature to asolid body below the liquidus temperature, it retains its amorphousstate and its transparency. If the molten glass is maintained for aperiod of time at temperatures near to but below the liquidustemperature, it slowly forms crystals which have chemical compositionsthat are normally slightly different from that of the parent glass. Therate of this devitrification is a function of the glass composition andthe difference between the temperature of the glass and the liquidustemperature. In the sheet glass process, devitrification shows asoptical defects in the LCD screen.

FIGS. 46 through 48 show in more detail the sources of the homogeneitydefects that may be produced at the downcomer pipe (7) to the inlet pipe(8) junction (14), as well as how different positions for the downcomerpipe affect production of these defects.

FIGS. 46A and 46B show the vertical position of the bottom (94) of thedowncomer pipe (7) at the same vertical position as the free surface ofthe glass (460). The length of the small arrows that indicate thestreamlines of flow (461) approximate the relative velocities of theglass flow at different locations in the downcomer pipe (7) and theinlet pipe (8). There is a vortex of glass flow (462) located at thebottom of the downcomer pipe (7) which is exposed to the factoryatmosphere at the free surface (460). The length of the arrows (462) inthe vortex is not scaled to the length of the arrows in the streamlinesof flow (461). The glass circulates in the vortex flow (462) field for along time which causes volatilization of some of the glass chemicals,thus changing the glass chemical and physical properties. The size ofthe vortex flow (462) is greater when the bottom (94) of the downcomerpipe (7) is below the free surface (460) of the glass.

Physical theory says that if the downcomer pipe (7) is circular and isperfectly centered in a circular inlet pipe (8), the vortex glass flow(462) between the downcomer pipe (7) and the inlet pipe (8) isstationary and does not bleed into the stream of glass flow (461). Therealities of manufacturing result in conditions where eitherperiodically or continuously a small portion of the glass from thevortex (462) bleeds into the main glass stream (461). The glass thatbleeds into the main glass stream has been subject to volatilization, islikely to have a different chemical composition and thus may create acord defect. Additionally, if the temperature of the glass in the vortexis below the liquidus temperature for a period of time, devitrificationdefects may be formed.

FIGS. 49A through 49D show examples of applying heat to the glass at thejunction (14) of the downcomer pipe (7) and the inlet pipe (8), to raisethe temperature of the glass in the vortex (462) above the liquidustemperature. This embodiment is useful when devitrification of the glassat the juncture (14) of the downcomer pipe (7) and the inlet pipe (8) isa concern or problem. It is particularly important when it is preferredor desirable to have the bottom end (94) of the downcomer pipe (7) at orbelow the glass free surface (460) and devitrification or cord is ahomogeneity defect problem. The applied heating solves thedevitrification problem. This may be done by placing heaters at the topof the inlet pipe (491) and/or the bottom of the downcomer pipe (492).

Another embodiment places heaters in the seal blocks (493). There arepreferably two seal blocks (493), which are symmetrical in shape, one ofwhich is shown individually in FIG. 49D. The seal blocks (493) aremanually set upon the insulated structure (133) to partially seal thefree surface (460) from the factory atmosphere. The important shape ofthe seal block is the semi-circular inner radius (494), which must fitclosely to the outside diameter (496) of the downcomer pipe (7) toprovide a partial seal between the free surface (460) of the glass andthe factory atmosphere. The seal block (493) shown is semicircular onits outer edge, however, the outer edge shape (495) may be rectangularor any other complex shape that would effectively seal the free surface(460) from the atmosphere at the top of the insulated structure (133).Some seal block configurations may include more than two seal blocks(493) in order to provide an adequate seal between the free surface(460) and the factory atmosphere and to accommodate any irregular shapeof the downcomer pipe (7).

U.S. Pat. No. 6,895,782, herein incorporated by reference, discussesglass flow in the vortex (462) between the downcomer pipe (7) and theinlet pipe (8). This patent describes how to shape the bottom of thedowncomer pipe (7), how to shape the inlet pipe (8) and how to adjustthem with respect to each other in the horizontal direction to controlthe time glass spends in the vortex (462) and what part of the glassstream into which it bleeds.

FIGS. 47A and 47B are related to FIG. 9C. The length of the small arrowsapproximate the relative velocities of the glass flow at differentlocations in the downcomer pipe (7) and the inlet pipe (8) for thecondition where the bottom (94) of the downcomer pipe (7) issubstantially above the glass free surface (90) to virtually eliminatethe condition of vortex flow. The arrows (471) show the streamlines offlow. The distance (93) shown is 0.25 times the inside diameter (476) ofthe inlet pipe (7). This distance (93) is in the approximate center ofan operating range which varies from 0.05 to 0.65 times the insidediameter (476) of the inlet pipe (7). The optimum distance (93) is afunction of the relative diameters of the downcomer pipe (7) and theinlet pipe (8) and the viscosity of the glass (10). Positioning thedowncomer pipe (7) relative to the inlet pipe (8) in this range is themost desirable in that it reduces the probability of generating cord,seed, and devitrification defects.

The length of the small arrows (481) and (487) in FIGS. 48A and 48Bapproximate the relative velocities of the glass flow at differentlocations in and between the downcomer pipe (7) and inlet pipe (8) forthe condition where the bottom (94) of the downcomer pipe (7) is locateda distance (483) of 1.00 times the inside diameter (476) of thedowncomer pipe (7) above the glass free surface (480). The glass stream(484) exiting the downcomer pipe (7) narrows as the glass acceleratestoward the free surface (480). The streamline arrows (487) in thisregion (484) are longer than the streamline arrows (481) below the freesurface (480), thus representing the difference in relative velocity.Where the stream enters the free surface (485), it creates vortex flow(482) similar to that in the case of the downcomer pipe (7) at or belowthe free surface (460) as shown in FIG. 46B. The length of these arrows(482) is not scaled to the length of the arrows in the streamlines offlow (481) and (487). At the entry point (485) of the stream (484) intothe free surface (480), air bubbles are trapped in the merging flowpaths of the descending stream (484) and the vortex (482). These airbubbles become seed inhomogeneities. The air bubble entrapment rateincreases as the distance (483) increases.

FIGS. 50A and 50B illustrate an embodiment of this invention whereby theshape of the inlet pipe (508) is altered in the vicinity of the freesurface (90) to further minimize the development of a stationary vortexat the juncture (14) of the downcomer pipe (7) and the inlet pipe (508).The inlet pipe (508) is flared outward by the angle (505) at theintersection (509) of the free surface (90) and the inlet pipe (508).The flared angle (505) produces a conical shape in a portion of theinlet pipe (508). Arrows (502) representing the flow in the vicinity ofthe intersection (509) of the free surface (90) and the inlet pipe (508)have a radial component which reduces the tendency to form a stationaryvortex ((462) in FIG. 46B and (482) in FIG. 48B). Angles (505) between10 and 50 degrees reduce the propensity of the stationary vortex toform. The addition of this conical section in the inlet pipe (508)increases the range of distances (93) over which the vortex ((462) inFIG. 46B and (482) in FIG. 48B) is minimized.

In summary the condition of FIG. 47, where little or no vortex (462) and(482) is generated at the downcomer pipe (7) to the inlet pipe (8)junction (14), is the most desirable. The condition of FIG. 46 where thedowncomer pipe is at or immersed in the free surface is tolerable withthe installation of heaters and/or with certain design and operationalconstraints, and the condition of FIG. 48 is to be avoided under allcircumstances.

Reducing Degradation of Sheet Glass Forming Apparatus

Referring now to FIGS. 11 through 16, another embodiment of the presentinvention controls the flow distribution of glass on the formingapparatus in a manner such that the degradation of the productionapparatus and the deformation of the forming structure that results fromthermal creep is compensated by thermal control of the glass flowdistribution.

U.S. Pat. No. 3,338,696 relies on a specifically shaped formingstructure to distribute the glass in a manner to form a sheet of uniformthickness. The basic shape of this forming structure is described indetail in U.S. Pat. No. 3,338,696. The sheet glass forming process isconducted at elevated temperatures, typically between 1000° C. and 1350°C. At these temperatures, the materials used for construction of theforming structure exhibit a property called thermal creep, which isdeformation of the material caused by applied stress. Thus, the formingstructure deforms under the stress caused by its own weight and thestress caused by the hydrostatic pressure of the glass in the trough.

The materials used in the construction of the other parts of the formingapparatus also degrade (warp, crack, change thermal properties, etc.) inan indeterminate way, which has an adverse effect on thicknessdistribution. The thickness control system of U.S. Pat. No. 3,682,609can compensate for small thickness errors, but it can only redistributethe glass over distances on the order of 5-10 cm. To significantlyeffect thickness distribution over the entire width of the glass sheet,the flow of the molten glass over the weirs must be controlled.

This embodiment of the invention solves this problem by introducing aprecise thermal control system to redistribute the flow of molten glassat the weirs, which is the most critical area of the forming process.This thermal control effectively counteracts the degradation of thesheet forming apparatus which inevitably occurs during a productioncampaign.

FIG. 12A shows the side view of the forming structure (9) with arrowsshowing the flow of molten glass (10) through the forming structure (9)to the side weirs (115). FIG. 12B shows a section through the center ofthe forming structure (9) which shows the different zones for thecontrol of molten glass (10) as it flows through the forming apparatus.Zone (121) is the flow in the trough (129) from the inlet end of theforming structure to the far end, zone (122) is the flow over the weirs,zone (123) is the flow down the outside of the forming structure, andzone (124) is the molten glass (11) being pulled off the root (116) andcooling into a solid sheet (12). The effect on the solid glass sheet(12) thickness caused by heating or cooling the molten glass (10) as itpasses through each zone is different. Adding energy to (raising thetemperature of) or removing energy from the molten glass (10) as itflows from the inlet end to the far end of the forming structure (9) inzone (121) produces concave or convex sheet thickness profilesrespectively. The period of the thickness profile changes effected inzone (121) is on the order of the length of the forming structure.

Changes to the energy flux to the molten glass (10) as it flows over theweirs (115) in zone (122) have a powerful effect on the resultant solidglass sheet thickness distribution. Localized cooling of the glass inzone (122) effectively produces a dam, which has a large effect on glassflow. This is an extremely sensitive zone, and any control strategyother than isothermal must be carefully designed. Zone (123) isimportant to return the glass to a uniform temperature distribution,substantially linear in the longitudinal direction, in order that thedrawing process at the root (116) is consistent. Differential cooling inzone (124) is the object of U.S. Pat. No. 3,682,609 and is effective inmaking small thickness distribution changes. Cooling at givenlongitudinal location affects the thickness at that location in onedirection and conversely to the glass on each side of the location. Theeffect is distributed over a distance on the order of centimeters.

FIGS. 11A and 11B show the prior art muffle (112), the top surface ofwhich is horizontal, whereas the forming structure (9) weirs (115) andthe top surface of the glass (10) flowing in the trough (129) of theforming structure (9) are sloped downward from the inflow pipe (8) tothe far end of the forming structure (9). Heat transfer at thesetemperatures between the muffle (115) and the glass (10) is primarily byradiation. Therefore, the distance between the muffle (115) and theglass (10) affects the distribution of the energy transferred because ofthe characteristics of radiation heat transfer. Heating elements in thechamber (119) are nominally equidistant (136) from the muffle (112).Therefore, each element (138) has substantially the same effect onenergy transfer from the element (138) to the muffle (112).

The distance (137) between the muffle (112) and the glass (10) at theinflow end is substantially less than the distance (139) between themuffle (112) and the glass (10) at the far end, therefore the heattransfer at the inflow end is more concentrated than the heat transferat the far end. The result is that a change in energy of a heatingelement (138) at the inflow end has a more targeted effect on thetemperature of the glass (10) than a change in energy of a heatingelement (138) at the far end. The use of heating elements (138) inchamber (119) to change the temperature and consequently the localizedflow rate of the glass (10) flowing in the forming structure (9) isdocumented and claimed in U.S. Pat. No. 6,748,765.

FIGS. 13A and 13B show an embodiment of this invention whereby the topof the muffle (132) is shaped more closely to the outside surface of themolten glass (10) that is flowing in the trough (129) and on the formingstructure (9). The muffle (132) is heated by heating elements (138) inheating chamber (131), which is encased in the insulated structure(133). The distance (137) between the muffle (132) and the glass (10) atthe inflow end is substantially equal to the distance (139) between themuffle (132) and the glass (10) at the far end, therefore the heattransfer at the inflow end is substantially the same as the heattransfer at the far end.

By designing the muffle (132) to conform closely to the outside shape ofthe molten glass (10) flowing in the trough (129) in the formingstructure (9), energy may be directed to targeted areas of the moltenglass (10), thereby effecting greater control of temperaturedistribution. The heating elements (138) in the heating chamber (131)have adequate power to balance the energy flux to the forming structure(9) and thus create suitable temperature conditions.

FIGS. 14A and 14B show an embodiment of this invention which effectslocalized cooling of the molten glass (10) as it passes over the weirs(115) in zone (122).

The muffle (132) configuration of FIGS. 13A and 13B is used. Air coolingtubes (142), similar in function to those air cooling tubes (141) thatare described in U.S. Pat. No. 3,682,609, are directed at the heatingchamber side of the muffle (143) just above the molten glass (10)flowing over the weirs (115). Localized cooling of the glass in thislocation effectively produces a localized dam, which has a significanteffect on the thickness distribution of the solid glass sheet.

FIGS. 15A and 15B show an embodiment of this invention whereby themulti-chamber muffle (156) is designed with separate heating chambers(151-155) to control the temperature of the molten glass (10) as itspasses through the various individual zones of the forming process.These zones (121-124) are described in FIGS. 12A and 12B. Themulti-chamber muffle (156) has five heating chambers (151-155). Heatingchamber (153), located over the top of the forming structure (9),affects the flow of glass from the inlet end to the far end of theforming structure (9) (zone (121)). The heating chambers (152) and (154)over the top of the weirs (115) affect the flow over the weirs (115)(zone (122)), and the heating chambers (151) and (155) on each side ofthe forming structure (9) are used to balance the temperaturelongitudinally (zone (123)). All the heating chambers (151-155) haveheating elements with adequate power to balance the energy flux to theforming structure (9) and thus create suitable temperature conditions.

FIGS. 16A and 16B show an embodiment of this invention which affectslocalized cooling to the molten glass (10) as it passes over the weirs(115). This is zone (122) shown in FIG. 12B. The multi-chamber muffle(156) configuration of FIGS. 15A and 15B is used. Specially designedradiant coolers (161), installed in heating chambers (152) and (154),have the ability to selectively cool the heating chamber side of themuffle surface (162) opposite the weirs (115). The radiant cooler hasmultiple adjustments (164) such that the temperature of its bottomsurface can be varied in the longitudinal direction. The distribution ofthe heat transfer between the radiant cooler (161) and the mufflesurface (162) is a function of the distance (163). By varying thedistance (163) between the cooling device (161) and the muffle surface(162), the cooling effect may be attenuated to adjust sensitivity.Although it is not illustrated, the cooling devices (161) arereplaceable during operation. The radiant coolers (161) couldalternately be inserted from the side instead of the top with a suitablechange in the design of the heating chambers (152), (153) and (154).

In an alternative embodiment, the air cooling tubes (142) of FIGS. 14Aand 14B could be used with the muffle (156) design of FIGS. 15A and 15B,and the radiant coolers (161) of FIGS. 16A and 16B could be used withthe muffle (132) configuration of FIGS. 13A and 13B.

Reducing Thickness Variations in the Glass Sheet

Referring to FIGS. 17 through 20, another embodiment of the presentinvention supports and compresses the forming apparatus in a manner suchthat the deformation that results from thermal creep has a minimumeffect on the thickness variation of the glass sheet. This embodimentintroduces a counteracting force to these stresses on the formingstructure in a manner such that the thermal creep which inevitablyoccurs has a minimum impact on the glass flow characteristics of theforming structure. The invention is designed such that thiscounteracting force is maintained through an extended period of theproduction campaign. Thus, sheet glass may be manufactured for a longertime with the same forming structure.

The refractory materials from which the forming structure and itssupport structure are made have high strength in compression and lowstrength in tension. Like most structural materials, they also changeshape when stressed at high temperature. This embodiment was developeddue to the material characteristics and how these characteristics affectthe manufacturing process.

There are two fundamental concepts in this embodiment of the invention.First, applying a force and/or moment to the ends of the formingstructure counteracts stress caused by the forces of gravity, thusminimizing the effect on molten glass flow caused by thermal creep.Second, the invention uses compression members shaped such that thermalcreep, to which the compression members are also subject, does notsubstantially alter the application of said force and/or moment.

FIGS. 17A and 17B illustrate the typical effects of thermal creep on theshape of the forming structure. FIG. 17A shows that the formingstructure (9) sags in the middle such that the top of the weirs (115)and the root (116) are now curved (171) and the trough bottom (117) hasa change in curvature (171). This curvature (171) causes the moltenglass (10) to no longer flow with constant thickness (172) over theweirs (115). This curvature (171) allows more glass to flow over themiddle of the weirs resulting in an uneven sheet thickness distribution.FIG. 17B shows how the hydrostatic force (174) from the molten glass(10) in the forming structure (9) forces the weirs (115) to move apartat the top. This allows more glass to flow to the middle of the formingstructure (9) making the thickness in the middle even greater.

FIGS. 18A through 18D show a sheet glass forming apparatus (180) asknown in the prior art. The forming structure (9) is supported by aninlet end supporting block (181) and a far end supporting block (182).The forming structure (9) is the equivalent of a beam, which is subjectto a bending stress from its own weight, from the weight of the glass inand on the forming structure, and from drawing forces. Because of thelow tensile strength of the forming structure material, a compressiveforce (183) is applied to the lower half of the forming structure (9) toforce the material at the root (116) of the forming structure (9) intocompression. Typically the inlet end support block (181) is restrainedin the longitudinal (horizontal) direction (175) and the compressionforce (183) is applied to the far end support block (182). The prior artconsiders only preventing tension at the root (116) of the formingstructure (9), and then only the stress at start-up. Littleconsideration is made for the effects on stress of the thermal creep ofthe forming structure (9) and its support blocks (181) and (182).

FIGS. 19A through 19D show an embodiment of a sheet glass formingapparatus (190) that has shaped end support blocks (191) and (192). Theinlet end shaped support block (191) is restrained in the longitudinaldirection (175). A compression force (193) is applied to the far endshaped support block (192). The shape of the support block is designedin a manner to produce a force distribution in the forming structure (9)to substantially counteract the effect of the weight of the formingstructure (9) and the molten glass (10). The applied force (193) is suchthat all material in the forming structure (9) is under substantiallyequal compression stress in the longitudinal direction (175). Thisstress causes the thermal creep to occur primarily in the longitudinaldirection (175) with little of the sagging shown in FIG. 17A. Theforming structure (9) gets shorter due to the equal compressive stressin the longitudinal direction (175). The shaped support blocks are alsosubject to thermal creep. The cross section of the shaped support blockis the same over substantially its entire length with equal compressivestress across its section. Thus as the shaped support block deforms fromthermal creep, it continues to apply substantially the same forcedistribution to the forming structure (9).

FIGS. 20A through 20D show an embodiment of a sheet glass formingapparatus (200) that has four shaped end support blocks (201), (202),(204), and (205). The inlet end has three shaped support blocks (201),(204), and (205), all of which have longitudinal compression forces(206), (207), and (208) applied. A compression force (203) is applied tothe far end shaped support block (202). The shape and loading of thesupport blocks (202) and (203) are designed to the same criteria assupport blocks (191) and (192) in FIGS. 19A-19D. The two upper shapedsupport blocks (204) and (205) are attached to the inlet end of theweirs and are angled inward such that they exert an additional force onthe weirs to counteract the affect of the hydrostatic forces which tendto spread the weirs apart.

In a preferred embodiment, short (10-25% of length) transition zones(not shown) are at the forming structure ends of the shaped supportblocks. In these transition zones, the cross-section of the shapedsupport block will change from that of the shaped support block to ashape that will suitably apply the design load to the forming structure.

Compression Loading of the Forming Structure

FIGS. 42A through 42D show a sheet glass forming apparatus (420) thatrepresents prior art. The design is primarily that described byCortright in U.S. Pat. No. 3,519,411. The forming structure (9) issupported by an inlet end support and compression block (421) and a farend support and compression block (422). The inlet end support andcompression block (421) rests on the inlet end structure (423) and isrestrained in the longitudinal (horizontal) direction (175) by anadjustment screw (424) and a sealing force (429) at the formingstructure (9) inlet. The far end support and compression block (422)rests on the far end structure (425), and a far end compression force(426) is applied to the far end of the forming structure by the supportand compression block (422) at the surface (427). The force (426) isgenerated by the far end force applicator (428) acting between thesupport and compression block (422) and the far end structure (425). Inone embodiment, the force applicator (428) is a force motor. The forceapplicator used in the prior art is an air cylinder that generates asubstantially constant force. Prior art considers only preventingundesirable tension at the root (116) of the forming structure (9).

A force motor as defined herein is a device that generates asubstantially constant force in a linear direction and maintains thatforce for the linear stroke required in the application. The tolerancefor variation of the force level is preferably plus or minus 5 percentor less over the full range of the stroke. The energy required tomaintain this force may be supplied by gravitational, pneumatic,hydraulic, or mechanical means. Some examples of force motors include,but are not limited to, an adjustable spring assembly, a mechanicaladjustment device that is constantly or periodically monitored andadjusted, an air cylinder, an air powered motor, a hydraulic cylinder, ahydraulic powered motor, a solenoid, an electric motor, or a weight andlever system.

Referring to FIGS. 43A through 43D, a substantial improvement over theprior art, discussed in U.S. Pat. Nos. 6,889,526, and 6,990,834, hereinincorporated by reference, uses multiple force applicators to enable thecompressive forces (426) and (436) at the root (116) of the formingstructure (9) to remain consistently at the desired levels throughoutthe production campaign.

FIGS. 43A through 43D show a sheet glass forming apparatus (430) wherethe weight of the forming structure is supported at the inlet end by theinlet end structure (433) at the surface (431). Additionally, it isconstrained horizontally by a small sealing compression force (429) atthe surface (439). The weight of the forming structure is supported atthe far end by the far end structure (435) at the surface (432). Thesurface (432) is designed to have very low friction in the horizontaldirection, thus contributing negligible force in the horizontaldirection. The inlet end compression force (436) is applied to thebottom portion of the forming structure by a compression block (437),which is designed to have low friction in the direction of the appliedforce. The inlet end compression force (436) is generated by the inletend force applicator (438). The far end compression force (426) isapplied to the bottom portion of the forming structure by thecompression block (434). The far end compression force (426) isgenerated by the far end force applicator (428). The far end compressionforce (426) must be slightly greater than the inlet end compressionforces (436) to compensate for the inlet pipe sealing compression force(429). The forming structure bottom compression forces (426) and (436)are applied with low friction and can be maintained at the same and/orany preprogrammed level throughout a production campaign. The forceapplicators (428) and (438) are preferably force motors.

Another substantial improvement over the prior art is shown in FIGS. 44Athrough 44D. Shown is a sheet glass forming apparatus (440) where theweight of the forming structure is supported at the inlet end by theinlet end support and compression block (421). Additionally, the inletend structure (445) constrains the forming structure horizontally by asmall sealing compression force (429) at surface (449). The weight ofthe forming structure is supported at the far end by the far end supportand compression block (422). The inlet end compression force (436) isapplied to the bottom portion of the forming structure by the supportand compression block (421). The inlet end compression force (436) isgenerated by the inlet end force applicator (448). The far endcompression force (426) is applied to the bottom portion of the formingstructure by the support and compression block (422). The far endcompression force (426) is generated by the far end force applicator(428). The far end compression force (426) must be slightly greater thanthe inlet end compression forces (436) to compensate for the inlet pipesealing compression force (429). The forming structure bottomcompression forces (426) and (436) are maintained at the same and/or anypreprogrammed level throughout a production campaign. The forceapplicators (428) and (438) are preferably force motors.

The force applicators apply forces in opposite longitudinal directions(175) to the respective support and compression blocks such that thebottom of the forming structure is in substantially greater compressionthan the top of the forming structure. In one preferred embodiment, thecompression stress in the bottom of the forming structure is between1.25 and 4 times the compressive stress in the top of the formingstructure. In another preferred embodiment, the compression stress inthe bottom of the forming structure is between 1.75 and 2.5 times thecompressive stress in the top of the forming structure. The bottom ofthe forming structure, which has greater resistance to thermal creepthan the top of the forming structure, is deformed longitudinally bythermal creep the same magnitude as the top of the forming structure.Consequently, any deformation of the forming structure that results fromthermal creep has a minimal effect on a thickness variation of the glasssheet.

An additional embodiment, which may be used with a prior art apparatus,periodically adjusts the inlet end adjustment screw (424) to compensatefor changes in the sealing force (429) as the forming structure deformslongitudinally by thermal creep during the production campaign. Thetorque on the adjustment screw (424) may also be monitored; however, thefriction will degrade the accuracy of torque as an indicator of thesealing force (429) actually applied to the forming structure (9). Thisembodiment of the invention is counter intuitive as adjusting theadjustment screw (424) in a direction to reduce the integrity of theglass seal between the inlet pipe (8) and the forming structure (9) willmake operating personnel nervous.

Effects of Surface Tension on the Sheet

In an alternative embodiment of the invention, the width and the angleof the inverted slope of the forming wedge may be changed to alter theeffect of surface tension and body forces on the narrowing of the sheet.In addition, the width and the inverted slope angle may be increased tomake the structure stiffer and thus more resistant to thermal creep.

FIGS. 21A through 21G show the prior art shape of the forming structure.The cross-section of the wedge shaped portion, FIGS. 21C through 21G, isuniform over the entire usable length of the forming structure. Thewidth of the forming structure (211) and the angle of the inverted slope(210) are identical at each section. As the molten glass (10) flows downthe vertical portion (211) of the forming wedge (9), the surface tensionand body forces have a minimal effect on the sheet width (212), whereas,when the molten glass (10) flows vertically down the inverted slopeportion (210) of the forming wedge, the surface tension and body forcesact to make the sheet narrower (213).

FIGS. 22A through 22G show an identical width of the forming structure(211) over its entire length, whereas the angle of the inverted slope(210) is the same in the center of the forming structure (FIGS. 21D-21F)and the angle of the inverted slope (220) at each end is reduced. Thisreduced inverted slope (220) has a counterbalancing effect on thesurface tension and body force stresses and thus reduces the narrowingof the sheet (223).

FIGS. 23A through 23G show the width of the forming structure (211) andthe angle of the inverted slope (210) being the same in the center ofthe forming structure (FIGS. 21D through 21F and FIGS. 22D through 22F),whereas the width of the forming structure (231) and the angle of theinverted slope (230) at each end are reduced. This reduced width (231)and inverted slope (230) have a counterbalancing effect on the surfacetension and body force stresses over the effect of FIGS. 22A through 22Gand thus further reduces the narrowing of the sheet (233).

FIGS. 24A through 24G show another embodiment of this invention, wherethe width of the forming structure (211) and (231) and the angle of theinverted slope (210) and (230) are the same as in the embodiment ofFIGS. 23A through 23G except that the angle of the inverted slope (240)at the center of the forming structure, FIG. 24E is substantiallygreater than the other inverted slopes (210) and (230). This greaterangle increases the section modulus of the structure making it stifferand thus less prone to thermal creep. Keeping the configuration of theends the same as FIGS. 23A through 23G has substantially the same effecton the surface tension and body force stresses as FIGS. 23A through 23Gand thus has little effect on the narrowing of the sheet (243).

Producing a Flat Sheet

U.S. Pat. No. 3,338,696 considers only the glass flow in the formingstructure and assumes that the drawn glass from the bottom of theforming structure will be of uniform thickness and flatness because ofthe uniform thickness of the flow of glass to the critical point ofsolidification. In practice, glass must be preferentially cooled acrossits width to create forming stresses during solidification that create aflat sheet. The present invention alters the forming stresses andcooling distribution such that the formed sheet is inherently flat.

FIGS. 25A through 25D show an embodiment of this invention where theshape of the root (116) of the forming wedge (259) is not straight butcontinuously curved convex upward (250) in the shape of a parabola. Thiscauses the glass that is drawn from the center of the forming wedge(251) to cool faster than the glass drawn from each edge (252) of theforming wedge. This imposes stress on the partially solidified glass(251) in the center of the sheet to cause the sheet to be flatter,having less warp.

The vertical dimension (257) of the parabola varies between 1% and 10%of the horizontal length (258) of the glass covering the formingstructure (259), with a preferred range of 3% to 5%. The angle of theinverted slope of the forming wedge at the inflow end and at the far end(254) of the forming structure (259) is the same as the angle of theinverted slope (255) at the center of the forming structure (259).

FIGS. 26A through 26D show another embodiment of this invention wherethe shape of the root (116) of the forming wedge (269) is a continuouslycurved convex upward (250) parabola as in FIGS. 25A through 25D, butwhere the angle (264) of the inverted slope of the forming wedge at theinflow end and at the far end of the forming structure (269) issubstantially less than the angle of the inverted slope at the center ofthe forming structure (269). FIG. 26A shows the parting line (263) thatdefines the break point where the top edge of the inverted slope startsis horizontal, whereas FIG. 25A shows the parting line (253) thatdefines the break point where the top edge of the inverted slope startsis parabolically curved.

The equation for the viscous force in glass flow is:

F=μ*dv/dx=μ*dv/dz*dz/dx  (1)

Where:

F=a viscous force

μ=absolute viscosity

v=velocity

Differentiating the equation that describes the force change is

dF=μ*d(dv/dx)+(dv/dx)*dμ  (2)

The equation for a parabola is:

z=k*x ²  (3)

where:

z=the vertical axis

x=the horizontal axis

k=constant of proportionality

The derivative of z with respect to x is:

dz/dx=2*k*x  (4)

Combining equations (1) and (4)

F=μ*dv/dz*2*k*x  (5)

Differentiating with respect to x and combining

dF/dx=2*k*(μ*x*d(dv/dz)/dx+x*(dv/dz)*dμ/dx+μ*dv/dz)  (6)

If μ is constant and dv/dz is small with respect to x then

dF/dx≈2*k*μ*dv/dz  (7)

Equation 4 shows that the shape change of a parabola with respect to xis a constant. Equation 7 shows that the change in force with respect tox is substantially constant if we can control the longitudinaltemperature distribution (x direction). Comparing the form of equation(7) with the form of equation (4), it is seen that the use of aparabolic shape is consistent with the equations of fluid flow.

Referring to FIG. 27A through 27E, from equation (2) and the assumptionsfor equation (7) the control of the glass temperature distribution inthe zone at the root (116) of the forming structures (259) and (269)where the glass sheet is formed is critical. The change in the glassviscosity (dμ) is a strong function of the glass temperature atdifferent locations in the formed glass sheet. In FIGS. 25A and 26A thisis the comparative temperatures at locations (251) and (252). Theprocess temperature at weirs (115) at the top of the forming structures(259), and (269) is higher than the temperature at the root (116). FIG.39D shows a typical temperature difference of 50 degrees centigradewhich is typical of a prior art shaped forming structure (9). The endsections (256) and (266) of the forming structures (259) and (269),respectively, extend lower into the forming zone than the center section(255) and (265), and therefore may be subject to greater heat loss. Alarge amount of the heat lost from the end sections (256) and (266) andcenter sections (255) and (265) is by radiation out of the bottom of theforming chamber (113) through the opening provided by the edges (271),(273), and (274) in the bottom doors (272). In the prior art (FIG. 27E),these doors (272) have a straight interior edge (271) and are movedlaterally in and out to control the radiation heat loss by making thegap (276) between the doors smaller or larger. In a preferred embodimentof this invention the interior edges (273) of the doors are shapedparabolically (273) to control the relative heat loss from the endsections (256) and (266) and center section (255) and (265) of theforming structures (259) and (269). In another preferred embodiment, theinterior edges (274) of the doors are shaped in a series of straightsections (274) to control the relative heat loss from the end sections(256) and (266) and center section (255) and (265) of the formingstructures (259) and (269). The exact shape of the interior edges (273)and (274) is preferably determined by heat transfer analysis and/orexperiment.

In the above paragraph it was assumed that the longitudinal temperaturedistribution at the root (250) should be a constant to comply withequation (7). In another preferred embodiment, it is desirable to havethe temperature at the ends (252) of the formed sheet be lower than thetemperature at the center of the formed sheet (251). This scenario wouldproduce a longitudinal force distribution per equation (6).

The shaping of the interior edges (271), (273), and (274) to control theradiation heat loss from the bottom sections of the forming structure(9) is not restricted to forming structures with a continuously curvedconvex upward (250) parabolic shape, but may be used with any shapeforming structure (9) where the longitudinal control of radiation heatloss is required.

Reducing Air Leakage

U.S. Pat. No. 3,338,696 relies primarily on careful design and matchingof materials to prevent any material cracks and openings. These cracksand openings are the sources of air leakage for both initial operationand for operation during the course of a manufacturing campaign. Thisembodiment of the invention provides individual pressure balancingtechnology such that even if a leakage path exists at start-up ordevelops during operation, a minimum quantity of air will flow throughthe leakage paths.

The glass sheet is formed by drawing the glass from the bottom of theoverflow forming structure. The molten glass is cooled and is solidifiedin a carefully controlled manner. The most desirable cooling phenomenonis radiation, which cools the glass substantially evenly through itsentire thickness. Convective cooling, which cools primarily the glasssurface, is also a factor. The convective cooling must be preciselycontrolled as when it is excessive, it has a destabilizing effect on thedrawing process. The observed destabilizing phenomenon is a cyclicvariation in sheet thickness as the sheet is drawn. This is termed“pumping” and is a phenomenon noted in virtually all glass downdrawprocesses.

The operating temperature of the forming zone of “The Overflow Process”is typically 1250° C. and is at the top of a chamber with an openbottom, typically 3 meters high, containing an atmosphere of hot air.Because of the approximately 3 meter column of high temperature air, theatmosphere in the zone where the sheet is formed has a pressure higherthan the pressure outside of the forming apparatus. Therefore, any crackor opening creates an airflow path whereby air flows into the openbottom of the chamber, up the chamber, and out the cracks or openings.When this leakage path is such that it substantially increases theconvective cooling in the forming zone at the root (116) of the formingstructure (9), a cyclic variation in the sheet thickness (pumping) isproduced.

For air to flow through an opening there must be a difference inpressure from one side of the opening to the other. This inventioninvolves adjusting the internal pressure in each of the major componentsof the forming apparatus such that the pressure difference across anyleakage path to the forming zone approaches zero. Therefore, if anopening either exists or develops, little or no air leakage will occuras there is negligible differential pressure to force airflow.

Referring now to FIGS. 28A through 32B, FIGS. 28A and 28B show coolingof the glass as it transitions from the molten state to the solid state.This process must be carefully controlled. This cooling process startson the lower part of the forming apparatus (9) just above the root(116), in the muffle zone (280), continues as the molten glass sheet(11) passes through the muffle door zone (114), and is substantiallysolidified by the time it leaves the transition zone (281). Thecontrolled cooling process continues in the annealer and pulling machinezone (282) to relieve internal stress in the solidified glass sheet(12).

Four embodiments for controlling forming chamber pressure differentialsare shown in FIGS. 29A through 32B. These are a) adding flow topressurize, b) restricting outflow, c) flowing to a vacuum, and d)encasement by a pressurizing chamber, respectively. Any of these controlmethods may be used to control the pressure in the muffle zone (280),the muffle door zone (114), the transition zone (281), or the annealingand pulling machine zone (282) depending on unique design requirements.An important objective, however, is to equalize the pressure on eachside of the membrane separating either the factory atmosphere or aheating zone or a cooling zone from the forming chamber. This inventionalso applies to implementations of “The Overflow Process” where the gasin the forming chamber is a gas other than air, i.e. nitrogen, etc.

More specifically, FIGS. 29A and 29B show an embodiment of the mufflezone (280) which shows air (290), which is preferably preheated,introduced into the muffle heating chamber (131) to make the pressure inthe heating chamber (131) equal to that in the adjacent forming chamber(113). The wall (132) separating the two chambers in the muffle isnormally constructed of many pieces and is therefore susceptible torandom leaks. Equalizing the pressure between the two chambers minimizesthe leakage flow.

FIGS. 30A and 30B show an embodiment of the muffle door zone (114),which includes an exit restriction (300) to the flow of air exiting eachmuffle door (301). The size of this restriction is varied to maintainthe air pressure inside the muffle door chamber (302) equal to the airpressure in the adjacent forming chamber (303). The flow of air into themuffle doors (301) through the cooling tubes (141) would normally beadequate to overcome any leakage paths and thus raise the internalpressure in the muffle door chamber (302) to the pressure of theadjacent forming chamber (303).

Referring to FIGS. 51, 52, and 53, another way to minimize the leakagefrom the muffle doors (301) through any joints or cracks is to preciselycontrol the cooling air mass flow into (511) and out of (517) the muffledoor chamber (302). FIG. 51 shows one embodiment of this invention wherethe device (512) controls the cooling air mass flow (511) into themuffle door chamber (302) and the device (516) controls the cooling airmass flow (512) out of the muffle door chamber (302). The total coolingair mass flow (511) into the muffle door (301) is regulated bymeasurement and the control device (512) as it flows to the individualflow adjusters (513), which proportion the cooling flow among theindividual cooling tubes (141), which are described in U.S. Pat. No.3,682,609, incorporated herein by reference. The cooling air exits themuffle door (301) through multiple exits (514) into the manifold (515)into the measurement and control device (516), which regulates theexiting of cooling air mass flow (517). In this embodiment, the coolingair mass flow into (511) and the cooling air mass flow out of (512) themuffle door (301) are regulated to the same value, therefore, there isno net air leakage from cracks and joints in the muffle door (301). Ifall the leakage paths external to the muffle door (301) are sealed, thenthere is no leakage from the front (518) of the muffle door (301) intothe internal chambers (113) of the forming apparatus. An additionalfeature of this embodiment is that the total cooling air mass flow iscontrolled to a constant value, which in turn stabilizes the energy lossin the muffle door zone (114).

In another embodiment, the device (512) measures the cooling air massflow (511) into the muffle door (301) and the device (516) measures theair mass flow out of (517) the muffle door (301) and controls the airmass flow out (517) to a value equal to the air mass flow in (511) asmeasured by the device (512). The settings of the individual flowadjusters (513) determine the cooling air mass flow rate (511).

In another embodiment, each individual flow adjuster acts as ameasurement and control device (523) whereby the sum of the measurementsequal the cooling air mass flow (511) into the muffle door (301). Thedevice (516) measures the air mass flow out of (517) the muffle door(301) and controls the air mass flow out (517) to a value equal to theair mass flow in (511) as determined by the sum of the measurements ofdevices (523).

FIGS. 31A and 31B show an embodiment of the transition zone (281), whichhas the cooling air at elevated pressure (310) entering the coolingchamber (311) and exiting (312) each of the transition coolers (313)into a regulated vacuum source (314). The large volume of air requiredfor cooling in the transition zone would normally raise the pressure inthe transition cooling chamber (311) above that of the adjacent formingchamber (315). A vacuum source (314) is therefore required to lower thepressure and is adjusted to equalize the pressure in the transitioncooling chamber (311) to the pressure in the adjacent forming chamber(315).

FIGS. 32A and 32B show an embodiment of the annealer and pulling machinezone (282), which includes a pair of pressure balancing chambers (320)on each end of the annealer and pulling machine (282). The pressure inthe balancing chamber (321) is adjusted to be equal to the pressure inthe annealing chamber (322). A chamber at each end was chosen becausethe bearings and adjustment mechanisms for the pulling rollers (111) areon the ends. Alternate configurations would be one single pressurebalancing chamber (320) encasing the entire annealer and pulling machine(282) or a multitude of individual pressure balancing chambers (320) aswould be required by particular design considerations.

Control of Convective Cooling

Referring back to FIGS. 51 through 53, the convective cooling in themuffle door zone (114) is controlled by accurate control of the coolingair flow into (511) and out of ((517) and (527)) the muffle door (301).The cooling of the glass as it is formed into a sheet at the root (116)of the forming structure (9) is dominantly by radiation to the frontsurfaces (518) of the muffle doors (301) in the muffle door zone (114).The temperature of the front surface (518) of the muffle door (301) isregulated primarily by the total cooling air mass flow (511). There isalso forced convective cooling of the glass in the muffle door zone(114) if there is air flow from the muffle chamber (113) to thetransition zone (281).

FIG. 52 shows a muffle door (301) with multiple vent openings (529) onthe top surfaces. These vent openings (529) are open to the mufflechamber (113) to provide air flow (528) for forced convective cooling ofthe glass in the muffle door zone (114). The cooling air mass flow (511)into the muffle (301) is measured and regulated by individual flowadjusters (523), which direct cooling air to the front surface (518) ofthe muffle door (301). The cooling air exits the muffle door (301)though multiple measurement and control devices (526) which regulate theexiting mass flow of cooling air (527). The convective cooling air massflow (528) exiting the muffle door is equal to the difference betweenthe cooling air mass flow into (511) and the cooling air mass flow outof (527) the muffle door (301). The convective cooling air mass flow(528) enters the forming apparatus chamber (113) through the ventopenings (529) and exits downward through the muffle door zone (114),providing a controlled quantity of forced convective cooling to themolten glass (10) that is being formed into a sheet (111) at the root(116) of the forming structure (9). As noted earlier in this text, anexcessive quantity of forced convective cooling may cause the glass flowrate to cycle (“pumping”) causing sheet thickness variation. Anadditional feature of this embodiment is that the total cooling air massflow is controlled to a constant value, which in turn stabilizes theenergy loss in the muffle door zone (114).

In another embodiment of this invention, the device (512) measures andregulates the cooling air mass flow (511) into the muffle door (301) andthe device (516) measures the air mass flow out of (517) the muffle door(301) and controls the air mass flow out (517) to a value equal to theair mass flow in (511) as measured by the device (512) minus the desiredforced convective cooling air mass flow (528).

The vent openings (529) between the muffle chamber (302) and the chambercontaining the sheet forming structure (113) must be large enough tosubstantially equalize the pressure between the two chambers such thatno air leaks through cracks and openings between the muffle door chamber(302) and the glass flowing off the root (116) of the wedged shapedsheet forming structure (9).

Compensation for Nonlinear Thermal Creep of Refractory

The refractory materials from which the forming structure (9) and itssupport structure are made have high strength in compression and lowstrength in tension. Like most structural materials they also changeshape when stressed at high temperatures. Recently available informationfrom U.S. Pat. No. 6,974,786, issued Dec. 13, 2005, entitled “SAGCONTROL OF ISOPIPES USED IN MAKING SHEET GLASS BY THE FUSION PROCESS”,herein incorporated by reference, defines the thermal creep materialcharacteristics of Zircon. Zircon is the presently preferred materialfor the construction of the forming structure. An analysis of how thesethermal creep characteristics affect the manufacturing process providedmotivation for the present invention.

The present invention includes embodiments that minimize the verticaldeformation of the forming structure, whereby the vertical deformationapproaches zero. This produces higher yield and delays termination ofthe production run for replacement of the forming structure.

FIGS. 33A-33D illustrates the principle parts of a typical “OverflowProcess” manufacturing system. The molten glass (10) from the meltingfurnace and forehearth, which must be of substantially uniformtemperature and chemical composition, enters the forming apparatus andflows into the sheet forming structure (9). The glass sheet formingapparatus, which is described in detail in both U.S. Pat. No. 3,338,696and in the applicant's U.S. Pat. Nos. 6,748,765, 6,889,526, 6,895,782,6,990,834, and 6,997,017 and patent application Ser. Nos. 11/006,251,11/060,139 and 11/184,212, is a wedge shaped forming structure (9).These patents and patent applications are incorporated herein byreference. Straight sloped weirs (115) substantially parallel with thepointed edge of the wedge (116) form each side of the trough (129) inthe forming structure (9). The trough bottom (117) and the sides (118)of the trough (129) are contoured in a manner to provide evendistribution of glass to the top of each side weir (115). The glass thenflows over the top of each side weir (115), down each side of the wedgeshaped forming structure (9), and joins at the root (116) to form asheet of molten glass. The molten glass is then cooled as it is pulledoff the root (116) to form a solid glass sheet (11) of substantiallyuniform thickness.

The usable width (331) of the formed sheet (11) is on the order of 70percent of the longitudinal length of the root (116) of the formingstructure (9) and is formed from the glass that flows over the middleregion (337) of the weirs (115). The glass flowing over the inlet endregion (336) and the far end region (338) of the forming structure (9)form the unusable end portions (334) and (335) of the formed sheet. Itis therefore most important that the middle region (337) of the formingstructure (9) be maintained as a uniform shape during the duration ofthe production campaign such that the thickness in the saleable portionof the sheet be of constant thickness.

FIGS. 34A through 34D illustrate the typical effects of thermal creep onthe shape of the forming structure when different compression forces areimparted to the bottom section of the forming structure (9) near theroot (116). FIG. 34A shows that, with no compression loading, theforming structure (9) sags in the middle such that the top of the weirs(115) and the root (116) are now curved (171) and the trough bottom(117) has a change in curvature (171). This curvature (171) causes themolten glass (10) to no longer flow with constant thickness (172) overthe weirs (115). More specifically, the curvature (171) allows moreglass to flow over the middle region (337) of the weirs (115) resultingin an uneven sheet thickness distribution. The forming structure (9) hasan initial length (346) as defined by the phantom lines (344) and (349).With no external loading the weirs (115) get shorter and the root (116)gets longer. At the neutral axis (341) the length of the formingstructure (9) does not change. The neutral axis (341), as used herein,defines the horizontal plane in the forming structure where there is nolongitudinal deformation when the forming structure (9) deforms underits own weight with no external forces. The neutral axis (341), as usedherein, also differentiates between the top section and the bottomsection of the forming structure (9); the top section or the uppersection of the forming structure (9) being vertically above the neutralaxis (341) and the bottom section or lower section being verticallybelow the neutral axis (341).

FIG. 34B shows that sagging of the forming structure is minimized underthe optimum compression loading (345) of the lower section of theforming structure (9) near the root (116). With optimal loading, boththe weirs (115) and the root (116) deform (shorten) to approximately thesame length (347). FIG. 34C shows that if too much load (342) is appliedto the lower section of the forming structure (9) near the root (116),the root (116) is compressed excessively, thus producing a convex upwardshape (342) to the trough weirs (115), the trough bottom (117), and theroot (116). The root (116) deforms considerably more than the weirs(115) as can be seen by the movement relative to the phantom lines (344)and (349). FIGS. 34A through 34C represent the effect of thermal creepover the same time period. FIG. 34D shows a forming structure (9), whichhas deformed a greater amount to length (348). This increaseddeformation is caused by imparting the correct load (345) for theincreased time of a substantially longer production campaign. Referringback to FIG. 33, this increased deformation has an adverse effect on theusable width (331) of the manufactured sheet as the usable sheet width(331) is a function of the width of the middle region (337) of theforming structure. The longitudinal deformation of the top of the middleregion (337) must be substantially equal to the longitudinal deformationof the bottom of the middle region (337) in order that the middle regionmaintain its shape, thus producing a usable width (331) manufacturedsheet (11) of uniform thickness. This increased deformation, over anextended period of time, may eventually cause termination of theproduction campaign because the usable sheet width (331) is notadequate.

The present invention recognizes the highly nonlinear characteristic ofthe thermal creep of the refractory material used for the constructionof the forming structure. The present preferred refractory material forthe forming structure is Zircon, whereas in the past other materialssuch as Alumina have been used for the forming structure refractory.FIG. 35 is a graph of Zircon's thermal creep coefficient as a functionof pressure and temperature, as defined by the data of U.S. Pat. No.6,974,786. To the inventor's knowledge, this data is not available inthe general literature. The data in FIG. 35 is derived from FIGS. 2B,3A, and 3B in U.S. Pat. No. 6,974,786. Since the raw data was notavailable, the curves of FIG. 35 represent a judgmental fit to the dataand therefore, the accuracy is sufficient only to represent trends andnot absolute accuracy. Extrapolation of the data was required to getpredictions of the thermal creep coefficients in the stress rangepredicted by the model.

FIGS. 36 and 37 show the thermal creep deformation as predicted by theprior art, while the creep deformation as determined by the presentinvention is shown in FIGS. 40 and 41. These figures are the result ofFinite Element Analysis (FEA) models of the various different boundaryconditions and material properties and represent expected thermal creepdeformations for a manufacturing period of 2 years magnified by a factorof 10. As discussed below, the variables considered in the analysisinclude the block shape, force loading, density, and thermal creepcoefficient/Young's Modulus, but not the weight of the glass. ALGOR®software was used for the Finite Element Analysis.

The FEA grid of the forming structure is shown in FIGS. 39A and 39B. Theforming structure is supported vertically at the first end and thesecond end at (391) and (392) respectively. The forming structure isrestrained longitudinally at (393). FIG. 39B shows that a half model wasused with symmetry being assumed at the vertical surface (394). The areato which the force is applied as a uniform pressure to the first andsecond ends is shown in FIGS. 39A and 39C as area (395).

The forming structure (9) modeled was for a forming structure rootlength of 2.00 meters. The finished dimensions of the refractory blockanalyzed are 2.20 meters long by 0.66 meters high by 0.20 meters wide.The bottom of the trough in which the glass flows is horizontal and theslope of the weir at the top of the forming structure is minus 5.73degrees. The included angle of the bottom of the root is 33.4 degrees.To the applicant's knowledge, the dimensions used do not preciselyrepresent actual dimensions used by any particular manufacturer usingthe overflow process; however, the dimensions are typical of what wouldbe selected by one skilled in the art.

The weight of the glass flowing in and on the forming structure was notincluded as part of the total load. Including the glass weight wouldhave a minimal effect on the amplitude of the deformation and negligibleeffort on the shape of the deformation. Including the glass weight wouldrequire the forces applied to the lower portions of the ends of theforming structure to be proportionally greater by the weight of theglass. The material density of the forming structure (9) used was 4,000kg/m̂3.

The thermal creep coefficient used for the calculations for FIGS. 36,37, and 38 was for the condition 250 psi and 1215° C. from FIG. 35. Alinear stress finite element analysis program was used to simulatethermal creep, the thermal creep coefficient multiplied by the timeinterval being analogous to the reciprocal of the Young's Modulus. Thevalue of the thermal creep coefficient used was 2e-8 in/in/hr/psi. TheYoung's Modulus used to simulate the thermal creep was 11.4e6 psi. Amagnification of the results by 40,000 for FIGS. 36, 37, 38, 40, and 41shows 10 times the deformation that would occur in 2 years. The forceapplied to the lower portions of the first and second ends was 0 (zero)lb. for FIG. 36, 2,250 lb. for FIG. 37, and 3,195 lb. for FIG. 38.

FIGS. 36, 37, 38, 40, and 41 are graphical results of a FEA. Using FIG.36 as an example, the shape of the shaded image (361) of the formingstructure shows the deformation of the forming structure for thespecific boundary conditions for the calculation made for FIG. 36,relative to the undeformed grid (362). The shading corresponds to thelongitudinal stress tensor X-X, the magnitude of which is defined in thelegend in the upper right corner of the corresponding figure.

FIGS. 36, 37, and 38 are the results of the predicted thermal creepusing a linear FEA for the boundary conditions. The contoured shadingrepresents the magnitude of the longitudinal stress tensor for a scaleof minus 450,000 to plus 450,000 newtons per square meter.

The shape of the shaded image (361) in FIG. 36 represents 10 times thethermal creep predicted in 2 years using a linear FEA if no compressionloading of the lower portion of the forming structure is used. The shapeof the shaded image (371) in FIG. 37 represents 10 times the thermalcreep predicted in 2 years using a linear FEA if compression loading ofthe lower portion of the forming structure of U.S. Pat. No. 3,519,411 isused. The shape of the shaded image (381) in FIG. 38 represent 10 timesthe thermal creep predicted in 2 years using a linear FEA to obtain asubstantially straight shape of the top of the forming structure (9) inthe middle region (387). Note that the middle region ((387) of theforming structure in FIG. 38) has a substantially straight shape,whereas both the inlet end region (386) and the far end region (388) areslightly curved convex upward. This shape maintains uniform flow overthe weirs (115) in the middle region (387) and changed flow over the endregion (386) and (388) weirs. The sheet formed has constant thickness inthe saleable middle portion, but different thickness and shape in theunusable end portions. The straight shape of the middle portion of thetop of the forming structure (9) is preferably obtained by having thedeformation or strain in the middle region of the upper portion of theforming structure (9) be substantially equal to the deformation orstrain in the middle region of the lower portion of the formingstructure (9).

The stress-strain model of the forming structure is that of a shortbeam. The stress is distributed and cannot be determined in the simplemanner of the stress-strain model of a long beam because a short beamhas more pronounced end effects than a long beam. The application of thelongitudinal compression forces to the lower portions of the formingstructure (9) can produce local stress concentrations like at locations(384) and (385) in FIG. 38. These particular stress concentrations areat the points of application of the longitudinal compression forces.

The stress and resulting thermal creep deformation (thermal creepstrain) of the forming structure is caused by gravitational forces whichproduce both the shear forces and the bending moment in the formingstructure. The vertical shear forces are caused by support of theforming structure at each end and are greater in the end regions (386)and (388). The bending moment is a maximum in the middle region of theforming structure. The bending moment produces the primary deformation;however, both the shear and the bending moment must be considered in theanalysis. Finite Element Analysis (FEM) is the preferred technology fordesign of the compression loading of the forming structure.

For some forming structure shapes, the specific combination of the shearand the bending moments make it desirable to implement the multiplestage compression force technology of U.S. patent application Ser. No.11/184,212.

FIG. 39D shows the assumed temperature distribution of the formingstructure in the nonlinear model. The top to bottom temperaturedifference is 50° C. The temperature used may not precisely representthe actual differential experienced by any particular manufacturer usingthe overflow process; however, the differential is typical of what wouldbe selected by one skilled in the art.

FIGS. 40 and 41 show the thermal creep deformation predicted by varyingthe material properties in a linear stress FEA in a manner to simulatenonlinear thermal creep. The contoured shading represents the magnitudeof the longitudinal stress tensor for a scale of minus 1,250,000 to plus250,000 newtons per square meter.

The shape of the shaded image (401) in FIG. 40 represents 10 times thethermal creep predicted in 2 years using a nonlinear FEA if compressionloading of the lower portion of the forming structure of FIG. 38 (alinear FEA) is used. It is the nonlinear analysis of the loading ofconfiguration of FIG. 38.

The shape of the shaded image (411) in FIG. 41 is a nonlinear analysisthat predicts the minimum thermal sagging condition. The force toproduce the predicted substantially zero vertical deformation of theforming structure is 6,075 lbs. versus 3,195 lbs. predicted by thelinear analysis shown in FIG. 38. The force of 6,075 lbs. to produce astable forming structure (9) shape is almost three times the force of2,250 lbs. that a linear analysis requires to produce zero longitudinaltension in the forming structure (9) per the claims of U.S. Pat. No.3,519,411. Note that the middle region (417) of the forming structure inFIG. 41 has a substantially straight shape, whereas both the inlet endregion (416) and the far end region (418) are slightly curved convexupward. This shape maintains uniform flow over the weirs (115) in themiddle region (417) and changed flow over the end region (416) and (418)weirs. The sheet formed has constant thickness in the saleable middleportion, but different thickness and shape in the unusable end portions.Because of the non-linear thermal creep properties of the Zirconrefractory, the straight shape of the middle portion of the top of theforming structure (9) is obtained by having the compressive stress inthe middle region of the bottom or lower portion of the formingstructure (9) be substantially greater than the compressive stress inthe middle region of the top or upper portion of the forming structure(9). These stress levels produce equal longitudinal deformation orstrain in the top and bottom portions of the middle region (417). Theapplication of the longitudinal compression forces to the lower portionsof the forming structure (9) can produce local stress concentrations atlocations such as in regions (414) and (415) in FIG. 41.

Nonlinear analysis predicts a longitudinal deformation (413) on theorder of 16 mm (413) because of the increased compression loadingrequired to produce a stable forming structure (9) shape. The loading ofthe linear analysis of FIG. 38 (U.S. Pat. No. 6,889,526) applied to anonlinear analysis produces a longitudinal deformation or strain (403)on the order of 8 mm, as shown in FIG. 40. The initial length (346) ofthe forming structure (9) would be made greater to compensate for thisincreased deformation if the width (331) of the usable sheet (11) wascritical and the present design width (346) was marginal.

The nonlinear analysis herein is a simplified example of the nonlinearanalysis that would be performed for a known production configuration.The grid is quite coarse and the magnitude of the thermal creepcoefficient was only iterated in the vertical direction. Also, moresophisticated FEA programs than that used herein are available which canautomatically iterate the thermal creep coefficient as a function ofstress and temperature level.

A review of the data of U.S. Pat. No. 6,974,786 shows a high variationin the test results of samples run at the same test conditions. Part ofthis variation is testing error; however, there is a high probabilitythat there is a significant variation in both the magnitude and slope ofthe thermal creep properties of different batches of Zircon refractorywith respect to temperature and stress. Thus, the forming structuresmade from different batches of material would have different thermalcreep properties. The analysis described herein can be performed for aforming structure (9) design using the average values of the thermalcreep properties; however, the deformation of individual formingstructures (9) in the production environment may differ from thepredicted deformation. To correct for this variation in deformation thefeedback control strategy of patent application Ser. No. 11/184,212,herein incorporated by reference, can be used.

In one embodiment the compression loading of the forming structure maybe implemented in the prior art manner of Cortright (U.S. Pat. No.3,519,411), where the forming structure (9) is restrained in thelongitudinal direction (175) by a fixed location adjustment bolt (424)at one end and the force applied by an active horizontal load (426) atthe other end. The apparatus in this embodiment is shown in FIG. 42Athrough 42D, but the force applied (426) has a much higher magnitudethan the force specified by Cortright. In an application of thisembodiment the adjustment bolt (424) is periodically adjusted tomaintain the loading at the desired magnitude per U.S. Pat. No.6,990,834.

In additional embodiments, shown in FIGS. 43 and 44, the forces (426)and (436) are applied as an active load at each end of the formingstructure per U.S. Pat. No. 6,990,834, herein incorporated by reference.

In another embodiment, illustrated in FIG. 45, a third active load (457)is induced at the far end of the forming structure to provide a sealingforce to overcome the hydrostatic pressure of the glass (429) enteringthe forming structure per U.S. Pat. No. 6,990,834. In this embodiment,the compression forces (436) and (456) have equal magnitude, butopposite direction.

A fundamental concept of this invention is to apply a force and/ormoment to the ends of the forming structure to counteract stress causedby the forces of gravity, thus virtually eliminating the effect onmolten glass flow caused by thermal creep.

Although the invention has been described with respect to variousembodiments, it should be realized this invention is also capable of awide variety of further and other embodiments.

Accordingly, it is to be understood that the embodiments of theinvention herein described are merely illustrative of the application ofthe principles of the invention. Reference herein to details of theillustrated embodiments is not intended to limit the scope of theclaims, which themselves recite those features regarded as essential tothe invention.

1-45. (canceled)
 46. A method for forming a glass sheet using animproved apparatus, wherein the apparatus includes a trough forreceiving molten glass that has sides attached to a wedged shaped sheetforming structure that has downwardly sloping sides converging at thebottom of the wedge such that a glass sheet is formed when molten glassflows over the sides of the trough, down the downwardly sloping sides ofthe wedged shaped sheet forming structure and meets at the bottom of thewedge, and wherein comprises the steps of: (a) providing at least oneinlet end compression block located at an inlet end of the formingstructure, wherein the inlet end compression block is positioned at abottom end of the forming structure; (b) providing at least one far endcompression block located at an opposite end of the forming structure asthe inlet end compression block, wherein the far end compression blockis positioned at the bottom end of the forming structure; (c) applyingan inlet end force with an inlet end force applicator to the inlet endcompression block such that a bottom of the inlet end of the formingstructure is deformed, by thermal creep, in a longitudinal direction;and (d) applying a far end force with a first far end force applicatorto the far end compression block such that a bottom of the far end ofthe forming structure is deformed, by thermal creep, in a longitudinaldirection; wherein the force applicators apply forces in oppositelongitudinal directions to the respective compression blocks such thatthe bottom of the forming structure is in substantially greatercompression than the top of the forming structure such that the bottomof the forming structure, which has a greater resistance to thermalcreep than the top of the forming structure, is deformed longitudinallyby thermal creep the same magnitude as the top of the forming structure;wherein the inlet end force applicator and the first far end forceapplicator consistently apply the corresponding inlet and far end forcesto remain at a desired level throughout a production campaign such thatany deformation of the forming structure that results from thermal creephas a minimal effect on a thickness variation of the glass sheet. 47.The method of claim 46, wherein a compression stress in the bottom ofthe forming structure is between 1.25 and 4 times a compressive stressin the top of the forming structure as measured in the middle region.48. The method of claim 47, wherein the compression stress in the bottomof the forming structure is between 1.75 and 2.5 times the compressivestress in the top of the forming structure as measured in the middleregion.
 49. The method of claim 46, wherein the inlet end forceapplicator is an inlet end adjustment screw that applies the inlet endforce.
 50. The method of claim 49, further comprising the step ofperiodically adjusting the inlet end adjustment screw to maintain anapplied force to the inlet end compression block such that a bottom ofthe inlet end of the forming structure is deformed, by thermal creep, inthe opposite longitudinal direction to the deformation at the bottom ofthe far end of the forming structure.
 51. The method of claim 50,wherein a compression stress in the bottom of the forming structure isbetween 1.25 and 4 times a compressive stress in the top of the formingstructure as measured in the middle region.
 52. The method of claim 51,wherein the compression stress in the bottom of the forming structure isbetween 1.75 and 2.5 times the compressive stress in the top of theforming structure as measured in the middle region.
 53. The method ofclaim 49, wherein the first far end force applicator is selected fromthe group consisting of a force motor; an adjustable spring; an aircylinder; a hydraulic cylinder; and electric motor; and a weight andlever system that applies the far end force.
 54. The method of claim 46,wherein the inlet end force applicator is selected from the groupconsisting of a force motor; and adjustable spring; an air cylinder; ahydraulic cylinder; an electric motor; and a weight and lever systemthat applies the inlet end force.
 55. The method of claim 46, whereinthe first far end force applicator is selected from the group consistingof a force motor; an adjustable spring; an air cylinder; a hydrauliccylinder; an electric motor; and a weight and lever system that appliesthe far end force.
 56. The method of claim 46, further comprising thestep of applying a force to a top of the far end of the formingstructure with a second far end force applicator to produce a sealingforce to counteract the hydraulic pressure of the glass flowing into theforming structure.
 57. The method of claim 56, wherein the second farend force applicator is selected from the group consisting of a forcemotor; an adjustment screw; an adjustable spring; an air cylinder; ahydraulic cylinder; an electric motor; and a weight and lever systemthat applies the force to the top of the far end of the formingstructure.
 58. A method for forming sheet glass using an apparatus,where the apparatus includes a trough for receiving molten glass thathas sides attached to a wedged shaped sheet forming structure that hasdownwardly sloping sides converging at the bottom of the wedge such thata glass sheet is formed when molten glass flows over the sides of thetrough, down the downwardly sloping sides of the wedged shaped sheetforming structure and meets at the bottom of wedge, and wherein themethod comprises the steps of: (a) providing at least one inlet endcompression block located at an inlet end of the forming structure; and(b) providing at least one far end compression block located at anopposite end of the forming structure as the inlet end compressionblock; (c) applying an inlet end force with an inlet end forceapplicator to the inlet end compression block such that a bottom of theinlet end of the forming structure is deformed, by thermal creep, in alongitudinal direction; and (d) applying a far end force with a firstfar end force applicator to the far end compression block such that abottom of the far end of the forming structure is deformed, by thermalcreep, in a longitudinal direction; wherein the inlet end compressionblock and the far end compression block are shaped to distribute forcein the forming structure to counteract the effect of the weight of theforming structure such that an applied force subjects the refractory inthe middle region of the forming structure to substantially equalthermal compression strain in the longitudinal direction from the top ofthe middle region to the bottom of the middle region that substantiallycounteracts the effect of the weight of the forming structure and themolten glass, thus reducing sag in the middle region, and wherein theinlet end force applicator and the first far end force applicatorconsistently apply the corresponding inlet and far end forces to remainat a desired level throughout a production campaign such that anydeformation of the forming structure that results from thermal creep hasa minimal effect on a thickness variation of the glass sheet.